V. Likodimos et al., COPPER MAGNETIC CENTERS IN OXYGEN-DEFICIENT RBA(2)CU(3)O(6- AN EPR AND MAGNETIC STUDY(X) (R=ND, SM) ), Physical review. B, Condensed matter, 54(17), 1996, pp. 12342-12352
EPR and magnetic results are reported for oxygen deficient, nonsuperco
nducting RBa(2)Cu(3)O(6+x) (R = Nd, Sm) compounds. The magnetic-suscep
tibility chi(T) and isothermal M(H) data are analyzed as the superposi
tion of the rare-earth R(3+) contribution with another strongly ferrom
agnetic (FM) contribution arising from FM copper clusters with large t
otal spin S. The rare-earth paramagnetic contribution in chi(T) and M(
H) are calculated using the results of consistent crystal-field analys
is (intermediate coupling wave functions, J-mixing effects) of Nd3+ an
d Sm3+ ions. The corresponding EPR spectra comprise an intense, almost
isotropic EPR line whose intensity I(T) exhibits a ferromagnetic beha
vior, while g(eff) and the linewidth Delta H-pp diverge al T < 10 K in
dicating the presence of slowly fluctuating ''internal'' fields. The o
rigin of the FM clusters is related to spin-polarized copper clusters
through oxygen holes in the Cu(1) or Cu(2) layers, while the ferromagn
etic interaction of the Cu2+(1) with the Cu2+(2) moments may be involv
ed in the low-temperature (Te 10 K) behavior of the EPR parameters. On
increasing the oxygen deficiency, the ferromagnetic contribution is d
rastically reduced and more isolated Cu2+ centers appear as shown by t
he corresponding EPR data. Exact simulation of the latter anisotropic
EPR spectra, shows that the anisotropic linewidths Delta H-i (i = x,y,
z) gradually broaden at low temperatures, while the intensity I(T) sho
ws antiferromagnetic behavior. EPR measurements on an ''aged'' Nd0.5Y0
.5Ba2Cu3O6+x sample revealed that the Cu2+ EPR spectrum intensifies wi
th time, a behavior probably related to oxygen ordering processes or t
o surface degradation effects. Analysis of the EPR resonance of Nd3+ a
nd Sm3+ ions in combination with the absence of the corresponding EPR
spectra indicate the presence of very fast spin-lattice relaxation of
the rare-earth ions.